This invention relates to a semiconductor test system for testing semiconductor integrated circuits such as a large scale integrated (LSI) circuit, and more particularly, to a power source current measurement unit provided in a semiconductor test system which is capable of measuring a power source current of a device under test with high speed and high accuracy. The power source current measurement unit of the present invention is advantageously applicable to measurement of a power source current IDD of a CMOS integrated circuit.
The power source (supply) current measurement unit of the present invention is used in a semiconductor test system for testing semiconductor integrated circuits such as LSIs (hereafter may also be referred to as xe2x80x9cdevice under testxe2x80x9d). Such a semiconductor test system performs mainly a functional test of the device under test, it also has a functionality for performing a DC parametric test in which DC voltage and DC current of the device under test are evaluated. The present invention is directed to a power source current measurement unit (DC test unit) for measuring a power source current of a device under test as a part of the DC parametric test.
The inventor of this invention has proposed a semiconductor test system based on notions of event (event based test system) which has an architecture different from a cyclized semiconductor test system (cycle based test system) widely used today. The power source current measurement unit of the present invention can be advantageously applied to the event based test system, however, it can also provide significant effects when used in the cycle based test system. Therefore, brief explanation will be made in the following as to the cycle based test system and event based test system.
FIG. 1A is a block diagram showing an example of basic configuration in the cycle based test system. In this example, a test processor 11 is a dedicated processor provided within the semiconductor test system for controlling the operation of the test system through a tester bus. Based on pattern data from the test processor 11, a pattern generator 12 provides timing data and waveform data to a timing generator 13 and a wave formatter 14, respectively. A test pattern is produced by the wave formatter 14 with use of the waveform data from the pattern generator 12 and the timing data from the timing generator 13, and the test pattern is supplied to a device under test (DUT) 19 through a driver 15.
The DUT 19 generates an output signal in response to the test pattern which is provided to an analog comparator 16 in a pin electronics 20. The output signal is converted to a logic signal by the analog comparator 16 with reference to a predetermined threshold voltage level. The logic signal is compared with expected value data from the pattern generator 12 by a logic comparator 17. The result of the logic comparison is stored in a failure memory 18 corresponding to the address of the DUT 19.
In such a cycle based test system, pattern data for producing test pattern must be described separately by waveform data, vector data, and timing data for each test cycle. Therefore, hardware and software involved in the cycle based test system become complicated, which makes it difficult to constitute a test system in such a way that each test pin is independent from the others.
FIG. 1B is a schematic block diagram showing an example of basic structure in an event based test system. Description for further details of the event based test system is given in the U.S. patent application Ser. No. 09/406,300 and U.S. patent application Ser. No. 09/259,401 filed by the same inventor of this invention.
In this example, the event based test system includes a host computer 42, a bus interface 43, an internal bus 45, an address control logic 48, a failure memory 47, an event memory consists of an event count memory 50 and an event vernier memory 51, an event summing and scaling logic 52, an event generator 24, and a pin electronics 26. A semiconductor device under test (DUT) 28 is connected to the pin electronics 26.
An example of the host computer 42 is a work station having a UNIX operating system therein. The host computer 42 functions as a user interface to enable a user to instruct the start and stop operation of the test, to load a test program and other test conditions, or to perform test result analysis in the host computer. The host computer 42 interfaces with a hardware test system through the system bus 44 and the bus interface 43.
The internal bus 45 is a bus in the hardware test system. An example of address control logic 48 is a tester processor which is exclusive to the hardware test system and is not accessible by a user. The address control logic 48 provides instructions to other functional blocks in the test system based on the test program and conditions from the host computer 42. The failure memory 47 stores test results, such as failure information of the DUT 28, in the addresses defined by the address control logic 48. The information stored in the failure memory 47 is used in the failure analysis stage of the device under test.
The address control logic 48 provides address data to the event memory configured by the event count memory 50 and the event vernier memory 51. The event memory stores event timing data describing each event (change point from xe2x80x9c1xe2x80x9d to xe2x80x9c0xe2x80x9d or from xe2x80x9c0xe2x80x9d to xe2x80x9c1xe2x80x9d) and its timing. For example, the event memory stores the timing data by two different types of data, one is integral part data showing an integer multiple of the reference clock, and fractional part data showing a fraction of the reference clock.
The event summing and scaling logic 52 is to produce data showing overall timing (delay time) of each event with respect to a predetermined reference point by summing the event timing data or modifying the timing data of each event by a scaling factor. The event generator 24 is to actually generate the test pattern (drive event) based on the overall timing data which is provided to the DUT 28 through the pin electronics 26. By comparing the response output signal of the DUT 28 with the expected data pattern (sampling event), the particular output pin of the DUT 28 is evaluated by the test system.
In the event based test system, since the event data for producing the test pattern is described only by the timing data of events, the data structure of the event data is significantly simplified. Therefore, the event based test system can be configured by a plurality of test pins each being independent from the others.
In the test system described above, the structure of the pin electronics for applying the test pattern to the device under test and receiving the output signal of the device under test is basically the same in either the cycle based test system or the event based test system. Generally, the pin electronics is also provided with a measurement unit for performing a DC parametric test. In FIG. 2, the pin electronics 26 which deals with the drive event (test pattern), sampling event (strobe) and parameters for the test unit is shown in more detail along with the event generator 24, pattern comparator 38 and the device under test (DUT 28).
The event generator 24 produces drive events (test pattern) which are provided to an input pin of the DUT 28 through the driver 35 in the pin electronics 26 with a predetermined amplitude and a slew rate. The event generator 24 further produces a sampling event which is provided to the analog comparator 36 as a strobe signal for sampling an output signal of the DUT 28. The output signal of the DUT 28 is converted to a logic signal by the analog comparator 36 when compared with predetermined reference voltages at the timing of the strobe signal. The logic signal at the output of the analog comparator 36 is compared with an expected logic pattern by the pattern comparator 38.
A DC test unit 37 performs DC parametric test for the device under test under the control of a controller such as the host computer. In the DC parametric test, ordinarily, the drive events supplied by the driver 35 to the device under test include a clock signal. The DC test unit 37 supplies a power source to a predetermined power pin of the device under test and measures a power source current, for example, IDD flowing through the power pin of the device under test. The measured value is transmitted to the host computer.
FIG. 3 shows an example of structure in a conventional DC test unit. This example only shows a power source current measurement unit for measuring a power supply (source) current of the device under test, for example, IDD of a CMOS device. The power source current measurement unit is formed with a DA (digital-to-analog) converter 71, an operational amplifier 72, a current buffer 73, a current measurement resistor 74, a differential amplifier 75, an averaging circuit 76, an AD (analog-to-digital) converter 77 and a buffer memory 78.
When the amount of voltage to be supplied, as a power source, to the device under test is specified by a controller such as the host computer, the DA converter 71 produces the specified voltage which is supplied to the device under test 28 through the operational amplifier 72. The operational amplifier 72 provides the power source to the device under test 28 through the current buffer 73 and the current measurement resistor 74.
As shown in FIG. 3, the power pin of the device under test 28 is a feedback point of a negative feedback loop formed in the measurement unit. Consequently, the input voltage at the operational amplifier 72 shows the voltage of the power source supplied to the device under test 28. The power source current is supplied by the operational amplifier 72 through the current measurement resistor 74 to the device under test 28. In this arrangement, the power source current flowing in the device under test 28 is measured by the voltage across the current measurement resistor 74 which is detected by the differential amplifier 75.
The output of the differential amplifier 75 is averaged by the averaging circuit 76 and supplied to the AD converter 77. The AD converter 77 samples an incoming signal by a sampling signal of predetermined repetition rate and converts the sampled voltage to a digital signal. The digital signal is stored in the buffer memory 78.
FIGS. 4A-4D are timing charts showing an operation of the conventional power source current measurement unit of FIG. 3. The test system supplies a clock signal of FIG. 4A to the device under test 28 through the driver 35 (FIG. 2), thereby starting the operation of the device under test 28. Although not shown, the test system also provides test patterns to the signal pins of the device under test 28. The power source current to the device under test such as a CMOS device changes in an impulse manner in the same repetition rate as the clock signal as shown in FIG. 4B.
The power source current is detected by the circuit configuration of FIG. 3 as a waveform of FIG. 4C which is provided to the AD converter 77. The AD converter 77 samples the measured current waveform of FIG. 4C by a predetermined sampling interval t and converts the sampled analog voltages to digital signals. The sampling interval t is, for example, several ten microseconds.
Since the power source current of the CMOS device largely changes depending upon the inner circuit conditions, the measured current waveforms provided to the AD converter 77 show a complicated pattern. Thus, in the conventional technology, to measure the current value accurately, the sampling operation shown in FIG. 4D is repeated to increase the number of samples, thereby improving the measurement accuracy by averaging the samples.
As has been foregoing, in the power supply current IDDmeasurement conducted by the conventional circuit, for improving the measurement accuracy, the same test pattern must be repeated many times to increase the number of samples, resulting in increase in the measurement time. If the measurement time is shortened, the measurement accuracy is significantly decreased especially when the current waveform changes rapidly. The conventional measurement circuit also involves a problem that it is not possible to specify as to which clock signal has caused the defect in the measured source current IDD.
Therefore, it is an object of the present invention to provide a power source current measurement unit for a semiconductor test system which is capable of measuring a power source current of a device under test with high speed and high accuracy.
It is another object of the present invention to provide a power source current measurement unit for measuring a power source current IDD of a device under test when the device under test is a CMOS device.
It is a further object of the present invention to provide a power source current measurement unit which is capable of measuring a power source current of a device under test within a short period of time with high accuracy by integrating measured values of power source current by an integration circuit and averaging the integrated value by the measured time.
It is a further object of the present invention to provide a power source current measurement unit which is capable of directly measuring an average value of power source current of a device under test by adjusting a repetition rate of a clock signal supplied from the semiconductor test system to the device under test by an arbitrary scaling factor and detecting the power source current in synchronism with the adjusted clock signal.
It is a further object of the present invention to provide a power source current measurement unit which is capable of detecting the relationship between the defective power source current value and a specific clock signal applied to the device under test by adjusting a repetition rate of a clock signal supplied from the semiconductor test system to the device under test by an arbitrary scaling factor and detecting the power source current in synchronism with the adjusted clock signal.
The power source current measurement unit of the present invention is provided in a semiconductor test system and is comprised of a DA (digital-to-analog) converter for generating a source voltage to be supplied to a device under test based on a digital signal received, an operational amplifier for forming a negative feedback loop and supplying the source voltage from the DA converter to a power pin of the device under test thereby supplying a power source current to the power pin through a current measurement resistor whose resistance is known, a voltage amplifier for amplifying a voltage representing the amount of power source current supplied to the device under test, an integration circuit for integrating an output signal of the voltage amplifier for a predetermined integration time, and an AD (analog-to-digital) converter for converting an output signal of the integration circuit after the integration time.
A clock signal for the device under test is supplied by the semiconductor test system for activating an operation of the device under test and an average value of the power source current resulted from the clock signal is measured by the power source current measurement. The integration circuit in the power source current measurement unit of the present invention includes a switch where the integration is performed by opening the switch for a predetermined time length.
Another aspect of the present invention is a semiconductor test system for evaluating a semiconductor device under test. The semiconductor test system of the present invention is comprised of test signal generating means for generating a test signal to the device under test based on event data prepared in advance, a pin electronics provided between the test signal generating means and the device under test and having a driver which provides the test signal to the device under test with predetermined amplitude and a comparator which compares a response output of the device under test with a predetermined reference voltage, and a power source current measurement unit provided either inside or outside of the pin electronics for measuring a power source current of the device under test.
The power source current measurement unit includes a DA converter for generating a source voltage to be supplied to a device under test based on a digital signal received, an operational amplifier for forming a negative feedback loop and supplying the source voltage from the DA converter to a power pin of the device under test thereby supplying a power source current to the power pin through a current measurement resistor whose resistance is known, a voltage amplifier for amplifying a voltage representing the power source current supplied to the device under test, an integration circuit for integrating an output signal of the voltage amplifier for a predetermined integration time, and an AD converter for converting an output signal of the integration circuit after the integration time.
The semiconductor test system supplies a clock signal to the device under test for activating an operation of the device under test. The semiconductor test system of the present invention further includes a scaling function which can modify timing data in the event data by an arbitrary factor. By adjusting the repetition rate of the clock signal from the semiconductor test system to the device under test by the scaling function, the clock signal can be set to a repetition rate equivalent to an operating speed of the power source current measurement unit. Accordingly, it is possible to synchronize the clock signal period with the open/close operation of the switch in the integration circuit in the power source current measurement unit, thereby enabling to measure the power source current corresponding to a particular period of the clock signal.
As a consequence, the power source current measurement unit of the present invention is capable of measuring the power source current of the device under test at high speed with high accuracy. Thus, the power source current measurement unit is advantageously applied to measuring a power source current IDD of a CMOS device. The power source current measurement unit of the present invention is capable of measuring the power source current of a device under test within a short period of time with high accuracy by integrating measured waveforms of power source current by an integration circuit and averaging the integrated value by the measured time.
The power source current measurement unit of the present invention is advantageously employed in an event based test system. The power source current measurement unit is capable of directly measuring an average value of power source current of the device under test by adjusting the repetition rate of the clock signal supplied from event based test system to the device under test by an arbitrary factor and detecting the power source current in synchronism with the adjusted clock signal. Further, since the power source current of the device under test can be measured in synchronism with the clock signal, it is possible to detect the relationship between the defective power source current value and a specific clock signal which caused the defect.